A US man has revealed why he decided to fire an employee over the use of a single "disrespectful" word, which insulted a fellow colleague.

The unnamed man, who goes by Seven (S) aints on Twitter, shared the saga on the social media platform earlier this week.

The boss, from Tallahassee in Florida, claimed one of his workers had referred to a transgender colleague as "it" — despite being warned against it.

The worker allegedly also underwent sensitivity training where it was explained why "misgendering" someone — or referring to them by the incorrect pronoun — was so insulting.

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However, the man persisted — until his boss stepped in.

The series of tweets coincided with Pride Month, an annual celebration of the LGBTI community.

"One of my employees called my other employee an 'it' because she's transgender. I fired him. Happy pride month, b**ch," Metro reported the man as posting.

"Being disrespectful towards employees or customers is detrimental to our work environment, productivity and bottom line. If they clearly (identify) as female use female pronouns. It ain't hard.

"I told him specifically don't call her an 'it'. He did … in front of other employees …. if I don't act that's a lawsuit waiting to happen.

"You work with this person 5-6 days a week. Learn their name or call them as the gender they clearly identify as. Easy to do my man. You smarter than 'it'."

The man's account is now private, and it is not known what industry the alleged incident occurred within, although the tweets have attracted an outpouring of support.

"This was beyond rude. Calling a transgender person 'it' is a deliberate attempt to demean that person because of what they are. It's an insult analogous to a racially or religiously offensive insult," one Twitter user posted, while another added: "Calling someone 'It' has always been rude. The correct pronoun for if you don't know the preferred pronoun is 'they'."